Industrial and domestic wastewaters require treatment before release into the surrounding environment. Multiple methods of waste water treatment systems involving abiotic and biotic processes are used for the same. Primary treatment involves physical removal of floating and suspended solids by filtration sedimentation, or floatation. Secondary treatment usually employs biological processes for the removal of dissolved impurities and other chemicals.
Various types of reactors are employed for remediation of potable water and industrial waste water. A reactor is an enclosed device that provides specific conditions for mediating and controlling chemical/biochemical reactions. These systems usually employ chemicals or microorganisms as catalysts to catalyze the reactions. The type of reactors include batch reactors, plug flow reactors, continuous flow reactors, sequential reactors, etc.
Abiotic reactors typically employ chemicals, radiation, high temperatures and pressures etc. for mediating reactions. There is increased potential for production of unwanted by-products, odors and corrosive gases. Also, the equipments and chemicals used may be expensive.
A wide variety of advanced oxidation processes (AOPs) have been reported for the removal of recalcitrant pollutants, namely, chlorophenols that include photocatalysis, photooxidation by hydrogen peroxide (H2O2/UV) and ozone (O3/UV), Fenton's type reactions, wet oxidation, direct oxidation by ozone, chlorine etc. However advanced oxidation processes are non-selective and attack chlorinated and non-chlorinated compounds with equal potency. Thus for a waste stream containing substantial concentration of biodegradable in addition to not so easily biodegradable chlorinated compounds, AOPs may not be economical. AOPs are especially expensive for small and medium scale industrial units. Abiotic processes such as adsorption on activated carbon or other adsorbents, air stripping, reverse osmosis are also expensive and merely result in phase transfer and not destruction of chlorophenols. Reductive dechlorination by zero-valent metals such as Mgo or Feo is very slow and often incomplete. On the other hand bimetallic systems such as Feo/palladium or Mgo/palladium can achieve complete dechlorination of recalcitrant pollutants such as chlorophenols, DDT, DDD, DDE. However, advantages of such high reaction rate and efficiency, mild reaction conditions and requirement for minimal follow-up treatment will be defeated if reactor design does not permit reuse and recovery of precious catalysts such as palladium.
Biological reactors employ microorganisms such as bacteria, fungi and algae to mediate transformation or degradation reactions. An important feature of a bioreactor is the ability of microbial cells to divide and reproduce. Very often the contents in the waste water itself serve as food to the microbes. However for industrial wastewaters (such as those discharged from chemical industries) easily biodegradable carbon source such as glucose and nutrients (sources of nitrogen and phosphorus) may need to be added to support growth of microorganisms and generate adequate quantity of biomass.
Bioreactors can be classified as aerobic, anoxic or anaerobic type depending upon the environmental conditions provided. The mode of growth of microorganisms within bioreactors may be attached or suspended. In suspended growth systems, such as activated sludge process (aerobic bioreactor), the contaminated water is treated in an aeration tank where a suspended flocs of microbial population aerobically degrades organic matter and produces CO2, H2O, and new cells. In anaerobic reactors such as upflow anaerobic sludge blanket (UASB) reactor, the bacterial population form granules that settle rapidly in the reactor. In attached growth processes microorganisms are immobilized on the surface of support materials such as plastic, acrylic etc. Bioreactors are widely used for their economical benefits and find extensive applications for treatment of domestic and industrial (such as pharmaceutical) waste waters.
Degradation of chlorinated pollutants by bacteria requires strict anaerobic conditions and very long acclimation (adaptation) period. Typically biological treatment of industrial discharges contaminated with chlorinated pollutants would require several days or weeks in anaerobic reactors. Concentration of the target pollutants and other conditions like pH, presence of salts and other co-pollutants and redox potential influences the efficiency of remediation. Methanogenic and sulfate reducing conditions (redox potential lower than −300 mv) enhances reductive dechlorination but denitrifying conditions inhibits reductive chlorination. A practical difficulty faced under field conditions is to provide favorable redox potential for reductive degradation of chlorinated pollutants. Aerobic degradation of chlorophenols like PCP involves ring cleavage prior to dechlorination and such products may be toxic. For example, the metabolite of aerobic biodegradation of pentachlorophenol, namely tetrachlorohydroquinone is more toxic than pentachlorophenol.
The limitation with bioreactors is the requirement of a growth medium at pH near neutral which contains biodegradable carbon source such as glucose or sucrose and nutrients (nitrogen and phosphate). Toxicity of the pollutants will either kill the bacterial cells or limits their growth (slow growth), and therefore require special design approaches. Low or very high ambient temperatures can significantly decrease biodegradation rates, resulting in increased maintenance time. Efficiency of bioreactors is reduced due to unwanted growth of problematic microorganisms that may preferentially colonize bioreactors. Disposal of resultant biosludge laden with adsorbed pollutants such as chlorinated compounds pose risk of ground water contamination due to leachate generation. Also incineration of the biosludge can lead to formation of toxic chlorinated dioxins. Air pollution controls may be required if there is volatilization of pollutants from bioreactors.
Catalysts
Remediation of potable water and industrial waste waters mainly containing chlorinated pollutants such as DDT, DDD, DDE and pentachlorophenol (PCP) is possible through reductive degradation reactions that are mediated by systems employing palladium (such as magnesium/palladium) as the reducing catalyst. The major hurdle to the field scale application of reductive conversion reactions mediated by systems employing palladium as the reducing catalyst is that the expensive reducing catalyst, namely palladium is either lost with the treated effluent or remains in the reaction system with residual solids in irrecoverable and unusable form. Thus, immobilized palladium which can be recovered and reused is preferred.
There are a number of commercially immobilized palladium catalysts available in the prior art. The choice of support material is extremely important and literature reveals various types of support materials like carbon, alumina, silica, zeolites, chitin, chitosan, amino acids and, metal oxides such as TiO2, MgO, ZrO2. Pd/C and Pd-alumina are some of the commercially available immobilized palladium catalysts. The main disadvantage of the present practice is expense, which precludes the field scale application. In addition there is no information on the application of these catalysts in reactors. Another disadvantage of the present practice is selectivity in action with respect to target pollutants. Also the catalytic activity and stability of immobilized palladium is strongly influenced by the type of support material used. It is also found that HCl is often released as the product of dehalogenation reaction and as a result of which the support materials are degraded and also undergo unwanted ion-exchange reaction with HCl leading to the loss of selectivity and activity of the catalyst. Thus, there is a need to explore alternative eco-friendly support materials for immobilizing palladium and design an indigenous reactor for reductive conversion reactions and for remediation of pollutant wastes.
U.S. Pat. No. 6,986,963 discloses employment of metallized bacterial cellulose in the construction of fuel cells and other electronic devices. The fuel cell includes an electrolytic membrane comprising of membrane support structure comprising of bacterial cellulose, an anode disposed on one side of the electrolytic membrane and a cathode disposed on an opposite side of the electrolytic membrane. The catalyst is disposed in or on the electrode support structure. However, the application of U.S. Pat. No. 6,986,963 is in electrical and electronic device manufacture where in the metallized bacterial cellulose is used as electrode. This application does not teach the use of bacterial cellulose immobilized and palladized in situ for hydrogenation reactions. It also does not teach the design and development of the palladized bacterial cellulose in reactors.
CA Patent No. 1156381 discloses a procedure for purifying wastewater containing chlorinated phenolic compounds. The purification is carried out biologically in a floating bed reactor which has been inoculated with a bacterial population decomposing chlorinated phenolic compounds. Microorganisms have disadvantages associated with their use. Requirement for a growth substrate further increased the costs of the purification. It was also found that toxic conditions affected survival of bacteria and reduced their activity. The present invention does not employ active microorganisms and thus tackles the limitations with biotic reactors.
HU Patent No. 9802650 discloses a bioreactor with cellulose fill. The microbial culture consisted of one or more strains of nitrogen removing bacteria that was established and sustained in the reactor by feed addition. It was found that following the removal of nitrate from water required further treatment with aeration, filtration.
In lieu of the advantages and disadvantages of processes available none of the treatment method posses all the desirable attributes such as high removal efficiency of chloroaromatic and chloroaliphatic pollutants, minimal follow-up treatment, economy, rapid rate of reaction and ease of operation. Clearly, there is a need for the design and development of an indigenous reactor that makes use of a highly efficient non-living catalyst such as palladium immobilized on an ecofriendly matrix for remediation of industrial discharges containing mixture of chlorinated pollutants. A robust, simple to operate and cost effective reactor that can mediate reductive conversion reactions in water containing toxic chlorinated pollutants such as PCP, DDT, DDD, DDE, chloroethylenes, chloroethanes or their mixture is the need of the hour.
The present invention mediates reductive degradation of pentachlorophenol, tetrachlorophenols, trichlorophenols, dichlorophenols and monochlorophenols and their mixture, reductive degradation of aliphatic chlorinated compounds such as chloroethanes and chloroethylenes (trichloroethylene (TCE), perchloroethylene (PCE) which are common contaminants in ground water, reductive degradation of DDT, DDD, DDE to the hydrocarbon end product, diphenylethane.
The present invention may also be applied for reduction of nitroaromatic compounds to amines (anilines). This is important since amine products such as anilines are industrially important intermediates for pharmaceuticals, polymers, herbicides, and other fine chemicals. Currently industries typically use zero valent iron for reduction of nitro groups to amines that is accompanied by the generation of huge quantity iron sludge (insoluble oxides of iron such as FeO, Fe2O3) that needs to be disposed. For example the rate of generation of iron sludge is ˜1.7 ton/ton of H-acid (an important intermediate in the textile dye manufacturing industry) produced. Moreover the iron sludge contains significant concentration of amine mass. The process of reduction involves extensive operations such as separation of iron sludge from product, the washing of iron sludge and dewatering of iron sludge. In case of the reactor (RCCR) developed through this invention this operation is very simple or not required.
In addition, the present invention is also useful in reductive decolorization of specific textile dyes (such as reactive black, drimarene dyes, remazole dyes), food colorants (such as sunset yellow and tartrazine). Currently there is no biological method that can decolorize mixture of dyes that are present in effluents discharged from textile dyeing mills. A specific microorganism can degrade only specific type of dye via the action of enzymes, azoreductases. Chemical oxidation processes such as ozonation and peroxidation are expensive for field scale application. Coagulation and flocculation processes generate chemical sludge which instigates disposal problems.